In this research, we investigate the problems that can occur in the channel size of oxide semiconductors, which are reduced by fabrication high resolution and large size displays. In particual, the post annealing is nessesary in the the oxide semiconductor TFT fabrication process. The purpose is to create a map that predicts the degreee of channel shortening length. Three types of aluminum-doped tin oxide (Al: ITZO) and two kinds of metal source/drain materials were used to manufacturing back channel etching (BCE) structure TFTs. The activation energy and the prefactor of diffusion were obtained by using the post-annealing treatment of each device. The XPS analysis of the structure before and after the post-annealing showed the change in amount of oxygen vacancies.